The present invention relates to portable radio communication devices, and more particularly to portable radio communication devices having signal processing circuits capable of being preset by the turning ON or OFF of the power supply.
Portable radio communication devices, such as radio paging receivers, usually use a single-cell battery (1.5 Volts, for example) as their power source to make them more compact and lighter. Also, such devices include a processing circuit for demodulating and detecting a calling signal. The processing circuit, which comprises C-MOS (Complementary-Metal Oxide Silicon) integrated circuits of less power consumption or a microprocessor having programmed procedures, requires a highly efficient DC-DC converter for boosting the voltage of the battery and supplying the boosted voltage to the processing circuit so that the detected calling signal can be processed at a high speed. A presetting circuit is further needed for setting the processing circuit in its initial state when a power source switch is turned ON.
Such a presetting circuit, composed of a differentiating circuit (comprising a resistor and a capacitor, for example) and other elements, usually has the disadvantage that, when the power source switch is turned ON, then OFF, and ON again within a short period of time, the output voltage of the differentiating circuit fails to reach the critical level at which the processing circuit can accept the presetting signal in response to the second turning-ON, because the capacitor cannot be sufficiently discharged during the short "OFF" period. Therefore, the processing circuit cannot be set in its initial state. Such quick repetition of turning ON and OFF the power switch often takes place when a person wants to make sure a paging receiver is ON (i.e., operative) before taking it out with him. As a result, the receiver often fails to perform normal receiving operation.